The present invention relates to a semiconductor device equipped with a gate insulator in an insulated gate transistor, as well as to a process for fabricating the semiconductor device.
In recent years, because of necessities for the suppression of variations in threshold voltages of transistors as well as for the suppression of the short-channel effect, there have been developed CMOS having a dual-gate structure using surface-channel type transistors that employ a gate containing N-type impurities for NMOS and a gate containing P-type impurities for PMOS. This has been reported, for example, in International Electron Devices Meeting 1996, pp. 555-558.
However, in an attempt to form the dual-gate structured CMOS with surface-channel type, transistors, there is a problem as follows. That is, when P-type doped polysilicon is used as a gate electrode, boron in the gate electrode penetrates through the gate oxide in heat treatment process for activation of impurities, reaching substrate silicon and making the threshold voltage of transistors largely changed.
For this reason, it has been reported in International Electron Devices Meeting 1990, pp. 429-432 that the penetration of boron can be suppressed by using nitride oxide as the gate insulator.
It has also been reported in IEEE Electron Device Lett. 10,141 (1989) that when polysilicon film containing no boron is used as the gate electrode, transistor characteristics and reliability are improved by introducing fluorine to the gate insulator.
However, it is reported in Symposium on VLSI technology, 1990, pp. 131-132 that using nitrided oxide film as the gate insulator would cause the mobility of transistors to be reduced, as compared with using oxide film.
In another aspect, as the surface-channel type P-type transistor, those in which polysilicon film containing boron as the P-type dopant is used as the gate electrode are the current mainstream. With such gate insulator given by silicon oxide and with fluorine contained in the gate electrode, fluorine accelerates the diffusion of boron, making it more likely that boron reaches the substrate silicon. This leads to a problem that the threshold voltage of the P-type transistor becomes more liable to vary.
In a further aspect, whereas gate insulated type transistors having a floating electrode and a control gate are used as nonvolatile memory, there has been a growing demand for transistors having a thinner gate insulator along with the scaling-down of devices. However, because a high electric field is applied to the gate insulator used in nonvolatile memory, the gate insulator would progressively deteriorate as its thickness decreases, which would cause a problem that the leak current increases. This deterioration begins to appear noticeably when the oxide film thickness becomes thinner than 10 nm, and tends to exponentially increase as the film thickness decreases.
This being the case, as a result of the inventor""s keen studies with a view to preventing boron from penetrating and diffusing into the substrate, and to avoiding the reduction of the mobility of the transistors, the present invention has been achieved.
In order to achieve the afore mentioned object, there is provided an insulated gate transistor having a gate electrode on a substrate with a gate insulator interposed therebetween, wherein the gate insulator composed of silicon and oxygen contains both nitrogen atoms and halogen atoms.
Because nitrogen and a halogen element are contained in the gate insulator, interface deterioration due to the introduction of nitrogen atoms to the interface is reduced so that the interface state density is reduced, and as a result, a successful interface can be formed. Also, because the halogen element contained in the interface forms a stable bond with silicon, even carrier injection with hot carriers or the like never causes the formation of dangling bonds. As a result of this, the insulated gate transistor has an effect of improvement in transistor characteristics and reliability. In particular, in the surface-channel type PMOS, to which the penetration of boron matters, oxide film containing nitrogen is used, whereas the use of oxide film containing nitrogen would cause a mobility deterioration to occur due to a deterioration of the interface characteristics. By an arrangement that halogen atoms having an interface-defect compensation effect are contained in this oxide film, the interface characteristics are improved. Although halogen atoms, when contained, would usually cause the boron penetration to be amplified, adding a sufficient concentration of nitrogen atoms makes it possible to suppress the mobility deterioration while suppressing the boron penetration.
In one embodiment of the present invention, nitrogen atom concentration of the gate insulator is not less than 1xc3x971020 cmxe2x88x923.
Because an insulator film having a nitrogen atom concentration of 1xc3x971020/cm3 or more is used for the gate insulator of the insulated gate transistor, boron contained in the gate electrode of the P-type transistor, in particular, does not diffuse into the substrate. Also, because any interface defects can be compensated by virtue of the halogen element contained in the gate insulator, the interface state density is reduced, the mobility is improved, and the transistor reliability is improved.
In one embodiment of the present invention, a source-and-drain region of the insulated gate transistor is stacked to upper than a channel portion.
For example, in a device as shown in FIG. 8, the contact hole for connecting the source-and-drain region and the upper connecting lines to each other does not need to be formed on the active region, and may be formed on the stacked layer extending up to on the device isolation region. This makes it possible to reduce the source-and-drain region width to the processible margin. That is, with the use of equipment that permits processing to a minimum processing size F, since the registration margin of photolithography for the upper pattern with the ground is generally about ⅓ F, the degree to which the source-and-drain region is ensured on the active region even with a maximum shift of registration, i.e., the gatexe2x80x94device isolation margin width has only to be about ⅔ F-F. Therefore, given a gate length of F, the distance from device isolation to device isolation is about {fraction (7/3)} F-3 F. Like this, when the device isolation is quite near to the gate electrode, the effect of abnormal diffusion of boron becomes more noticeable due to the stresses of the gate electrode and the device isolation. By applying this invention, the diffusion of boron can be inhibited without causing any deterioration of transistor characteristics. Besides, the transistor reliability can also be improved.
In one embodiment of the present invention, the insulated gate transistor comprises a floating gate electrode and a control gate electrode provided on the floating gate electrode with an interlayer insulator interposed therebetween. Because the transistor of a structure having a floating gate electrode and a control gate electrode has a necessity that a high electric field be applied thereto, the reliability of the gate insulator is of particular importance.
In particular, in regions where the film thickness of the gate insulator is not more than 10 nm, there arises a problem that the leak current flowing through the insulator film increases after the application of the high electric field. In this invention, the increase of the leak current in the gate insulator can be suppressed by the halogen element contained in the insulator film.
In one embodiment of the present invention, the halogen atom is fluorine. In particular, fluorine atoms, by virtue of being small in atomic radius, can improve the transistor characteristics without disturbing the bonded state of the atoms in the insulator film. Also, because fluorine and silicon can obtain a stable bond by virtue of large bond energy therebetween, an insulator film superior in reliability can be formed.
In one embodiment of the present invention, film thickness of the gate insulator is not less than 0.5 nm and not more than 5 nm. Even when the film thickness of this gate insulator is not less than 0.5 nm, a stable film formation is enabled by this invention. Also, in regions where the film thickness of the gate insulator is not more than 5 nm, boron would diffuse in the gate insulator with the occurrence of boron penetration when no nitrogen is contained in the gate insulator, whereas the boron penetration does not occur in this invention because nitrogen atoms are contained. Thus, it becomes possible to improve the transistor characteristics by the halogen element.
Also, there is provided process for fabricating an insulated gate transistor having a gate electrode on a substrate with a gate insulator interposed therebetween, comprising: a step for forming silicon oxide containing nitrogen atoms as the gate insulator; and a step for introducing a halogen element to the silicon oxide containing the nitrogen atoms. In this case, as the gate insulator, silicon oxide containing nitrogen is formed by reacting at temperatures of about 700-1200xc2x0 C. nitrogen monoxide gas, dinitrogen monoxide gas or ammonia gas on the silicon oxide. In this process, reaction temperature and reaction time are controlled so that nitrogen concentration in the silicon oxide becomes 1xc3x971020/cm3 or more. After that, the halogen element is introduced into the insulator film by using nitrogen trifluoride, nitrogen trichloride or other gas containing fluorine or chlorine, which is a halogen element. It is noted here that the halogen element may also be introduced into the gate insulator by ion-implantation of fluorine or chlorine in later steps. In the case where the halogen element is introduced with a gas, the optimum value of the halogen element contained in the insulator film is controlled by controlling the reaction temperature and reaction time; or in the case where the halogen element is introduced by ion implantation, the optimum value is controlled by controlling the injection dose. As a result, silicon oxide having a nitrogen concentration of 1xc3x971020/cm3 or more and containing a halogen element can be formed.
In one embodiment of the present invention, the step for forming silicon oxide containing nitrogen atoms comprises a step of forming silicon oxide and a step of nitriding the silicon oxide. First, silicon oxide is formed with an oxygen atmosphere or a water vapor atmosphere. Then, the silicon oxide is nitrided with nitrogen monoxide, dinitrogen monoxide, ammonia or other gas, by which silicon oxide containing nitrogen is formed. By forming silicon oxide containing nitrogen in this way, nitrogen-containing silicon oxide uniform in both film thickness and nitrogen content can be formed within the wafer surface.
In one embodiment of the present invention, the step of forming the silicon oxide containing nitrogen atoms is a step of forming the silicon oxide by using nitrogen monoxide. With the use of nitrogen monoxide, nitrogen-containing silicon oxide that has been controlled in nitrogen content can be formed by a single-step process.
In one embodiment of the present invention, the step of forming the silicon oxide containing nitrogen atoms is a step of forming silicon oxide with dinitrogen monoxide and then nitriding the silicon oxide with nitrogen monoxide or ammonia gas.
First, by forming nitrogen-containing silicon oxide with dinitrogen monoxide, nitrogen-containing silicon oxide which is thinner in film thickness due to a slow oxidation rate can be formed with good control. Then, by nitriding with the use of dinitrogen monoxide or ammonia gas, the nitrogen concentration can be enhanced.
In one embodiment of the present invention, the step of introducing a halogen element is a step of ion implantation of fluorine.
Because of fluorine""s high diffusion rate, fluorine can be easily introduced into the insulator film by injecting fluorine into the gate electrode and performing heat treatment therewith. Also, because of fluorine""s small atomic radius, transistor characteristics can be improved without disturbing the bonded state of atoms in the insulator film. Further, because fluorine and silicon can obtain a stable bond by virtue of large bond energy therebetween, an insulator film superior in reliability can be formed.